One hint to remember if dampness appears along a stretch of wall above a skirting board is that it may be caused by the plaster simply having come away from the wall. The moisture-content of the air in the gap created – which need be only the thickness of a cigarette paper – condenses when the heat of the room works its way through the plaster.
Cut back a 60cm 2ft square area of damp wall and apply a new coat of plaster. Leave it for a few weeks. If it stays dry, cut back the remainder of the damp wall surface. And replaster. If not. Cal1 in a professional to check the damp course.